This application is based on the patent application No. 11-324948 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
This invention relates to an actuator using shape memory alloy and method for controlling the actuator.
2. Prior Art
The shape memory alloy possesses a characteristic such that once it is set to a predetermined shape at a temperature in a predetermined high temperature range, say, 60xc2x0 C. and the shape thus set is memorized, then even if the shape memory alloy is transformed into a shape different from the memorized shape at a lower temperature in a normal temperature range, say, 20xc2x0 C., it transforms to the memorized shape on heating to a temperature above the transformation temperature thereof, say, 60xc2x0 C.
There is known an actuator which utilizes the above characteristic and which has an actuating member constructed of a shape memory alloy. In this known actuator, for example the actuating member is formed using a wire of a shape memory alloy, and a predetermined first shape, e.g., a shrunk shape, is memorized beforehand in the actuating member at a predetermined high temperature region, while at a lower, normal temperature region, an external force is applied to the actuating member with a biasing spring or the like to transform the actuating member into a second shape, e.g., an extended shape, different from the first shape and a driven member is connected to the actuating member, which driven member is set at a second position.
In this state, if the actuating member of the shape memory alloy is heated to a temperature above the transformation temperature thereof, the actuating member transforms to the memorized first shape thereof, i.e., the shrunk shape, from its second shape, or extended shape, whereby the driven member connected to the actuating member can be shifted from the second to the first position.
Then, when the actuating member of the shape memory alloy is cooled to a normal temperature, it is transformed again into its second shape, or extended shape, with an external force, say, the force of a spring, whereby the driven member connected to the actuating member can be restored from its first position to its second position, i.e., its initial position.
When an actuating member formed using a shape memory alloy is supplied with an electric current and is heated, there occurs a displacement corresponding to the electric current. This is because the quantity of heat generated in the actuating member of the shape memory alloy at a certain current value and the quantity of heat radiated to the ambient air of the actuating member has been balanced and the actuating member is held at a certain temperature. Thus, there is a close relation between the temperature and displacement of the actuating member.
Due to the above temperature displacement characteristic of the actuating member constructed of a shape memory alloy it turned out that the actuator formed by the shape memory alloy involved the following two problems to be solved.
The first problem is that if the ambient temperature exceeds the transformation temperature of the shape memory alloy, the actuating member transforms to its memorized shape and causes an operation which is not intended, even when the actuator is not to be operated. As long as the actuator is held at an ambient temperature which is a transformation temperature of the shape memory alloy, the transformed state of the actuating member to its memorized shape is continued, which is inconvenient.
As a countermeasure, in a device having an actuating member formed of a shape memory alloy, a transformation starting temperature of the shape memory alloy is set higher than an operation ensuring temperature range of the device. According to this method, however, since there is a wide difference between the transformation starting temperature of the shape memory alloy and a heating start temperature (generally a normal temperature), a considerable time is required for the actuating member of the shape memory alloy to be heated up to the transformation starting temperature and there arises a time lag from the output of an operation start signal to the device until the start of actual operation.
The second problem is that a drive unit using a shape memory alloy adopts a construction wherein an actuating member formed of the shape memory alloy is urged, for example pulled, with a biasing spring or the like. Therefore, in the initial state in which the actuating member of the shape memory alloy is not heated with an electric current, the actuating member is kept extended by the spring. When a reference position at the start of operation of the drive unit is set at an intermediate position of displacement induced in the actuating member, it is necessary to induce a displacement by heating the actuating member from the initial state in which the actuating member is not heated, up to the reference position as an intermediate position of displacement, thus causing a time lag, which is inconvenient.
It is a primary object of the present invention to provide an actuator using a novel shape memory alloy and capable of actuating a driven member quickly in response to an operation start command signal, as well as a control method for the actuator.
It is another object of the present invention to provide an actuator using a novel shape memory alloy and capable of diminishing a time lag from the output of an operation start command signal until the start of movement of an actuating member of the actuator, which time lag is based on the difference between a transformation start temperature of the shape memory alloy and a heating start temperature, thereby allowing the actuating member to start deformation quickly, and hence capable of actuating a driven member quickly, as well as a control method for the actuator.
It is a further object of the present invention to provide an actuator using a novel shape memory alloy and capable of being used for actuating an optical element which corrects an image shake caused by hand-shake of a camera.
It is a still further object of the present invention to provide an actuator using a novel shape memory alloy and capable of being used for actuating a component of a camera.
Other objects of the present invention will become apparent from the following detailed description of the invention taken in connection with the accompanying drawings.